A borderline case between meso and stable C1:: an axially chiral, yet configurationally semi-stable biphenyl with two oppositely configured [10]paracyclophane portions

The synthesis of the bi[10]paracyclophanes 2 and 4 from the meso-configured bioxepine 3 is described. These compounds are stereochemically remarkable: the biaryl axis that connects the constitutionally identical, but oppositely configured planar-chiral paracyclophane portions, is configurationally semi-stable. Thus, 2 is an unprecedented borderline case of a (planar-chiral)-(axially chiral)(planar-chiral) molecule that is right in between a meso-compound (as a macroscopical result of the-albeit slow-rotation about the central C,C-bond) and Cl-symmetry (with respect to the existence of separable-even though configurationally unstable-discrete atropo-enantiomers). Despite their restricted configurational stability, these atropo-enantiomers were resolved on a chiral phase at 5 degrees C and were stereochemically assigned by LC-CD coupling, in combination with quantum chemical CD calculations. (c) 2005 Elsevier Ltd. All rights reserved.